A parameterization of subgrid scale convective cloud vertical mixing has been developed in the National Oceanic and Atmospheric Administration Aeronomy Laboratory three-dimensional regional chemistry model. The parameterization is evaluated by comparing model results (with and without the cloud mixing parameterization) with observations of O3, NOx, and NOy made onboard the National Center for Atmospheric Research Sabreliner aircraft during the 1990 Rural Oxidants in the Southern Environment campaign in Alabama. Our studies show that model results with and without the cloud transport significantly differ from each other when convective clouds are present, implying that cloud exchange is an important process in detemining the distributions of trace species. Our studies also show that model results with the cloud transport parameterization are in much better agreement with the aircraft observations than those without. ¿ American Geophysical Union 1994